The present invention relates to power supplies for External Electrode Fluorescent Lamps (EEFLs).
Conventional fluorescent lamps and neon tubes employ electrodes sealed within the lamp, and to which power connections are made. These conventional lamps have low slope impedance around the normal running voltage of the lamp. Because of this, a constant current supply is used to feed the lamps. Each lamp must have its own constant current supply, unless the lamps are connected in series.
An EEFL does not have an electrode inside the lamp. Instead a metal end cap is used, which surrounds the end of the tube, and to which power is supplied. The voltage field around each end cap allows the internal gas in the lamp to become ionized, and produce an electrical discharge between the ends of the lamp. The capacitance of the metal end caps to the ionized gas provides an impedance to current flow, and which can be used to control the current flowing in the lamp.
A constant voltage source is typically used for EEFLs with the voltage frequency set to such a level that the required lamp current is achieved.